Matt Merkel Hess, Adobe Stock - Vjom, Caleb Kraft
After about 10 minutes, reinforce the cast with
plaster-gauze bandages. Add salt to the water
for dipping the bandages to speed drying time.
In Figure
D
, you can see I’m also holding a hair-
dryer to speed the drying process.
After the plaster gauze stiffens, carefully
demold the cast from your face. Alginate starts to
break down within 12 to 18 hours; best practice is
to make your next mold within 4 hours.
2. PLASTER POSITIVE
The plaster-backed alginate cast should be quite
stiff, but it can be backed with more clay so that
it doesn’t bend or deform. Putting this all in a
cardboard box will give you extra support and
help contain any plaster drips.
After preparing your mold, mix plaster
according to the manufacturer’s direction and
pour it into the mold to cast a positive (Figure
E
). You can use any quality plaster from
U.S.Gypsum; I mix No. 1 Pottery Plaster at
10 parts plaster to 7 parts water by weight. I
wouldn't use plaster of Paris, as you want a really
fine cast that will capture all the detail.
After the plaster hardens (about an hour),
remove it from the alginate mold. If necessary,
plaster can be scraped, cut, and sanded, so this
is your chance to get the mold how you want it
(Figure
F
and
G
). You might be able to use the
alginate cast again, but the heat of the plaster
curing can alter the alginate.
The plaster cast needs to dry. This takes 1 to
2 weeks by air drying, or you can speed it up by
putting it in a low oven or hot box. Make sure
to keep cured plaster under 150°F or you risk
ruining your mold.
3. TERRA COTTA BAKING DISH
After the plaster positive is dry, the next step
is making the baking dish. I used a traditional
low-fire red terracotta clay, but you could use just
about any water-based clay.
You will also need access to a kiln; most
community studios will allow you to fire by paying
by volume, typically 3¢–8¢ per cubic inch. If your
dish is around 10"×6"×6", it’ll cost about $11–$29.
PROJECTS: Halloween
NOTE: Air-dry clays and polymer clays like
Sculpey can't be used in the oven to bake bread,
so don't use them.
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